Process for reducing corrosion and composition of matter of reduced corrosiveness towards ferrous metals



Patented Apia, 23, 194E PROCE SS FOR REDUCING COREQSION AND COWOSITION 0F MATTER 0F REDUCED COBROSIVENESS TOWARDS FEBBOUS RETAES Herman A. Beekhu'is, Jr., lietersburg, Va, and

William De Forest Macomber, Clay, N. '32., assignors to The Solvay Process Company, New York, N. K, a corporafion or New York I No Drawing.

Application March 8, 1937,

Serial No. 125mm 13 Claims.

The present invention relates to methods of reducing corrosion of ferrous metals by -ammonium nitrate solutions containing free 'ammonia. 5 In recent processes for the ammoniation of superphosphates, use is made of solutions containing large percentages of ammonium nitrate and also free ammonia. The provision of methods which reduce or prevent the corrosion of apparahis in which these solutions are handled constitutes a major problem in the commercial use of these solutions, since, when ammonium nitrateammonia solutions, of the concentrations used, are shippedin tank cars of the usual construction, 35 rapid corrosion occurs and the tank soon'is not usable. Also corrosion occurs in apparatus constructed of ferrous metal in which the solutions are handled.

It is an object of this invention to provide a w ferrous metals by corrosive solutions of ammonium nitrate containing free ammonia, so that it is commercially practicable to employ apparatus of these metals for the storage and transportation of such solutions.

It is a further object of this invention to provide solutions of ammonium nitrate containing free ammonia which but slowly corrode ferrous metals.

According to the present invention, the ap-' paratus which may be a storage tank, tank car, conveying pipe, measuring tank or the like, is constructed of the ferrous metals commonly used for this purpose, such as low carbon steels, for example structural steel, cast iron and wrought iron, which are subject to severe corrosion by ammoniacal solutions of ammonium nitrate..

We have discovered that by dissolving any material containing divalent sulfur linked to carbon and a chromate in an ammoniacal-ammonium nitrate solution the corrosion of ferrous metals by the solution is reduced to a very low rate and protection against corrosion of the metal by the solution is consistently obtainai. The thio alcohols, such as ethanthiol (CaHsSH) the thio ethers, such as ethyl sulfide [(CaHs) 2S] the organic thio acids and their salts, for instance the thiocyanates, the thio analogues of carbonic acid,

and the thio analogues of the carboxylic acids, preferably the alkali metal thiocyanates, which are also called sulfocyanates and sulfocyanides (including ammonium thiocyanate) method for reducing the rate of corrosion of potassium xanthate and thio-acetic acid CHaC E 7 respectively; and the thiamides, for instance the thio analogues of the amides of carbonic acid and the thio analogues of the amides of the carboxylic acids, such as thio-urea [(NHa) 2C=Sl and thio-acetamide respectively, are examples of the sulfur compounds suitable for use in carrying out this invention.

When both a compound containing divalent sulfur and a chromate are dissolved in the ammonium nitrate-ammonia solution the effect of these materials in reducing corrosion of a ferrous metal by the solution is not merely additive. For example, active low carbon steel in contact with a solution containing 60 parts ammonium, nitrate, 20 parts ammonia and 20 parts water, was corroded'at the rate of .67 inch per year. With 0.1% NH4SCN added to the solution, the corrosion was reduced to 0.012 inch per'year. 0.1%

mime-21120 added to the ammonium nitrate-ammonia-water solution had little efiect in reducing the corrosion of the active steel; it was corroded at the rate of 0.56 inch per year. But by adding to the solution 0.1% NHiSCN and 0.1% NaaCrzOvZHaO, there was no measurable corrosion of the steel, which became passive in contact with the solution. As a possible explanation of the Joint action oi the above inhibitors and a chromate in the solution, it would appear that the inhibitors prevent immediate attack on the steel bythe solution without interfering with the action of the chromate on the steel to passiiy it. The action of many inhibitors in preventing corrosion or rusting of a metal is apparently due to the inhibitor i'orming a protective coating on the metal. In the case of the present invention, it the inhibitors used act in this manner, the coating they form on the inetalappearsnottobeoianaturemchthat the chromate dissolved in the ammonium nitrateammonia solution cannot reach the metal surface to act upon it. Further, the above data show the presence of the inhibitor made it possible for the chromate in the solution to passify the metal. Cast iron has been found to be not completely passified in carrying out the process of this invention, as was the case with steel; but even with cast iron, it has been found that the corrosion rate is consistently reduced by the presence of both the inhibitor and the chromate as compared with the rate when but one of these materials is present. The inhibitor appears to limit the immediate attack of the ferrous metal by the solution and the chromate serves to build up a passified condition of the metal.

Our invention has specific application to aqueous solutions of ammonium nitrate containing free ammonia in which the ammonium nitrate constitutes about 50% or more of the solution (for example, 40% to 80%) and in which the ammonium nitrate is dissolved in 5% or stronger aqua ammonia, preferably in an aqua ammonia of about 25% or greater strength, due to the relatively high rates of corrosion exhibited by these solutions.

This invention also has specific application to contacting, at temperatures below about 50 C., ferrous metals with solutions of ammonium nitrate and free ammonia.

In order to incorporate a chromate in the solution, any suitable material may be added to the solution, such as chromic acid, a chromate or a dichromate. These materials, when dissolved in the ammonium nitrate solutions here involved, appear to be full equivalents, having equal solubility therein on the basis of CrOs content and being of equal merit in reducing the rate of corrosion. Thus, when chromic acid, sodium, potassium or ammonium chromate or sodium, potassium or ammonium dichromate are added to individual portions of a solution containing ammonium nitrate, ammonia and water in the proportions 60--2020, the following solubilities are found for each of these materials, calculated as CIOsZ Salting out Weight temperature, percent C. Ci'O:

The amount of chromate incorporated in the aolutions may range from a trace upward, though )rdinarily not more than 0.17% ClOa is employed. )ur preferred range is from 0.02% to 0.07% CrOa )f the weight of the solution.

The quantity of the sulfur compounds added nay vary from a very small percentage upward. When ammonium thiocyanate or thiourea is add- :d to a solution containing 60 parts ammonium iitrate, 20 parts ammonia and 20 parts water vhic'h is contacted with low carbon steel, the hiocyanate or thiourea is preferably added in ;he amount of about 0.01% to about 2%. This alueis illustrative of the relative magnitude of he preferred quantities of these compounds to be idded to the solutions when used with the various errous metals.

In order that our invention may be more clear- 57 understood, the following examples typical of preferred methods of procedure are given by w'a' of illustration:

Example L-To a solution containing 60 parts ammonium nitrate, 20 parts ammonia and 20 parts 'water, 0.1% of sodium thiocyanate and about 0.07% sodium dichromate, calculated as CrOa, are added. The solution is then introduced into a container made of low carbon steel or cast iron and stored in such a container. Very little corrosion of the carbon steel or cast iron occurs during long period of contact with the solution.

Example II.--To a solution containing 60 parts ammonium nitrate, 20 parts ammonia and 20 parts water is added 0.05% thiourea and 0.05% sodium dichromate, which is equal to 0.035% sodium dichromatecalculated as CrOa. The solu- 7 tion is then introduced into apparatus constructed of cast iron and steel. Corrosion of such an apparatus in. contact with the solution described is but 3% of the rate attained in the absence of the thiourea and chromate.

This application is in part a continuation of our copending application Serial No. 726,735,

filed May 21, 1934.

We claim:

1. The process of reducing corrosion of ferrous metals by a solution of ammonium nitrate containing free ammonia, which comprises dissolving in said solution a chromate and a compound containing divalent sulfur linked to carbon in amounts suflicient to materially decrease the rate of corrosion of a ferrous metal by the solution, and contacting said solution with said ferrous metal. 1

2. The process of reducing corrosion of ferrous metals by a solution of ammonium nitrate containing free ammonia, which comprises dissolving in said solution an alkali metal thiocyanate and a chromate in amounts sufficient to materially decrease the rate of corrosion of a ferrous metal by the solution and contacting said solution with solving in said solution a chromate in amount such that the solution contains 0.02% to 0.17%

of chromium compounds, circulated as CrOa, and

a compound containing divalent sulfur linked to carbon in amounts suificient to materially decrease the rate of corrosion of a ferrous metal by the solution, and contacting said solution with said ferrous metal.

' 5. The process of reducing corrosion of ferrous metals by a solution containing about 50% or more ammonium nitrate dissolved in about 25% or stronger aqua ammonia, which comprises dissolving in said solution about 0.01% to about 2% of an alkali metal thiocyanate and a chromate in amount such that the solution contains between 0.02% to 0.07% of chromium compounds,

circulated asCrOs, and contacting said solution with said ferrous metal.

6. The process of reducing corrosion of ferrous metals by a solution containing about 50% or more ammonium nitrate dissolved in about 25% of thiourea and a chromate in amount such that.

the solution contains between 0.02% to 0.07% of chromium compounds, circulated as C103, and contacting said solution with said ferrous metal.

'7. The process of reducing corrosion of ferrous metals by a solution of ammonium nitrate containing free ammonia which corrodes said ferrous metals, which comprises dissolving about 0.01% to about 2% of an alkali metal thiocyanate in said solution, incorporating a chromate in said ammonium nitrate solution in amount such that the solution contains between 0.02% to 0.17% of chromium compounds, calculated as mm, and contacting said solution with said ferrous metal.

8. The process of reducing corrosion of ferrous metals by a solution of ammonium nitrate containing free ammonia which corrodes said ferrous metals, which comprises dissolving about 0.01% to about 2% of thiourea in said solution, incorporating a chromate in said ammonium nitrate solution in amount-such that the solution contains between 0.02% to 0.17% of chromium compounds, calculated as (Iron, and contacting said solution with said ferrous metal.

9. As a new composition of matter for handling and shipping in apparatus constructed of ferrous metal, a solution containing free ammonia, ammonium nitrate, a compound containing divalent sulfur linked to carbon, and a chromate, said solution containing said compound containing divalent sulfur linked to carbon and said chromate in. amounts sumcient to materially decrease the rate of corrosion of a ferrous metal by the solution.

10. As a new composition of matter for han-' dling and shipping in apparatus constructed of ferrous-metal, a solution containing free ammonia, ammonium nitrate, a thiocyanate, and a chromate, said solution containing said thiocyanate and chromate in amounts sufficient to materially decrease the rate of corrosion of a ferrous metal by the solution.

11. As a new composition of matter for handling and shipping in apparatus constructed of ferrous metal, a solution containing free ammonia, ammonium nitrate, thiourea, and a chromate, said solution containing said thiourea and chromate in amounts sufiicient to materially decrease the rate of corrosion of a ferrous metal by the solution.

12. As a new composition of matter for handling and shipping in apparatus constructed of ferrous metal, a solution containing free ammonia, ammonium nitrate, about 0.01% to about CERTIFICATE OF CORRECTION.

Patent No. 2, 198 151.

April 25, 191,0.

HERMAN A. BEEKHUIS, JR. ET AL. It is hereby certified that error appears in the printed specification of the above..number ed patent requiring correction as follows: Page 2, sec-. 0nd column, lines 57 and '(l, and page}, first column, line 5, claims ll, 5,

and 6 respectively, for the word "circulated" read -calculated--; and that the said Letters Patent should be read with this correction therein that the same -may conform to the record of the case in the Patent Office.

Signed and sealed this 14th day of June, A. D. l9l.lO.

(Seal) Henry Van Arsdale, Acting Commissioner of Patents.

CERTIFICATE OF CORRECTION. Patent No; 2,19 ,1 1. April 25, 19ho.'

HERMAN A. BEEKHUIS, JR., ET AL.

It is hereby certified that error appears in the printed specification of the abovehmnbered 'patent requiring correction as follows: Page 2, sec-. 0nd column, lines 57 and 71, and page 5, first column, line 5, claims ll, 5, and respectively, for the word "circulated" read --calculated; and that the said Letters Patent should be read with this correction therein that the same; 'may conform to the record of the case in the Patent Office.

Signed and sealed this hth day of June, A. D. l9).|.0.

fienry Van Arsdale, (Seal) Acting Commissioner of Patents. 

